Analytical Methods--Recommended procedures for calibrating dissolved oxygen meters
March 14, 1979
QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 79.10
Subject: Analytical Methods--Recommended procedures for
calibrating dissolved oxygen meters
Dissolved oxygen meters have been in routine use by the Water
Resources Division for a number of years for measuring dissolved
oxygen (DO) concentrations in fresh and saline waters. However up
to this point there has been no documentation of recommended
procedures for calibrating these meters. Several calibration
procedures, based largely on manufacturers' recommendations and on
instructions given at various WRD training courses,are in current
use within the Division. These procedures vary widely from
district to district.
In an attempt to develop a standardized and recommended DO meter
calibration methodology the Quality of Water Branch requested the
California District to conduct a detailed evaluation of several
calibration procedures. The evaluation was carried out over
temperatures ranging from 0.5 to 30 deg. C and at altitudes
ranging from sea level to over 8,000 feet. Based on the results of
this evaluation the attached method describing four DO meter
calibration procedures was prepared. The four procedures are: (l)
air calibration chamber in water, (2) calibration by Winkler
titration, (3) calibration with air saturated water, and (4) air
calibration chamber in air. All four procedures will give
comparable results (within +/- 0.1 mg/L when compared with a
Winkler DO measurement) provided the procedures are followed
carefully.
Use of one of the above four procedures is recommended for
calibration of DO meters used in collection of data for WRD
programs. Of the four procedures, the air calibration chamber in
water (procedure l) is preferred. It is the most convenient to use
and the most accurate under a variety of conditions. In situations
where this calibration procedure cannot be used, one of the other
three should be used.
Note 6 on page 5 of the DO calibration method points out a problem
in using the YSI DO probe model 5739 with the air calibration
chamber. This probe has a pressure compensating diaphragm which
will not fit inside the air calibration chamber and can lead to
erroneous calibration. Quality of Water Branch Technical
Memorandum No. 79.08(B), dated March 7, 1979, describes a
modification which can be made to the air calibration chamber to
accomodate the DO probe and diaphragm, thereby eliminating this
problem.
Any questions or comments regarding these DO meter calibration
procedures should be addressed to the Chief, Quality of Water
Branch, with copies to your Regional Water Quality Specialist.
R. J. Pickering
Chief, Quality of Water Branch
Attachment
WRD Distribution: A, B, FO, PO
Oxygen, dissolved, electrometric, polarographic probe (I-1576-78)
Parameters and Codes: Oxygen, dissolved (mg/L): 00300
Oxygen, dissolved (percent of saturation): 00301
1. Application
1.1 This method may be used to analyze waters and wastewaters
containing between 0.1 and 20 mg/L of dissolved oxygen.
2. Summary of method
2.1 Dissolved oxygen (DO) is determined by an electrode system
consisting of a sensor or probe containing a gold cathode and
silver anode surrounded by an electrolyte solution of potassium
chloride. A thin oxygen-permeable membrane is stretched across the
end of the probe to isolate the electrolyte and sensor elements
from the environment. When a potential of 0.8 volt is applied
across the cell, oxygen is reduced at the cathode causing current
to flow in an amount proportional to the partial pressure of
oxygen at the external membrane surface. Inasmuch as oxygen is
consumed at the gold cathode, the oxygen pressure inside the
membrane is effectively zero, and the force causing oxygen to
diffuse through the membrane is therefore proportional to the
absolute pressure of oxygen outside the membrane. A linear
relationship exists between the cell current and external oxygen
pressure or concentration of oxygen in the sample solution.
Four procedures for calibrating the oxygen meter are given. They
are listed in order of preference.
3. Interferences
3.1 The membrane is permeable to gases, other than oxygen.
Halogens, hydrogen sulfide, sulfur dioxide, and helium interfere.
Hydrogen sulfide poisons the electrode.
4. Apparatus
4.1 Oxygen meter, Yellow Springs Instrument Co., Model no. 54 or
no. 57, equipped with model 5739 probe, or equivalent.
4.2 The oxygen meter selected should meet the following general
requirements:
Range of dissolved oxygen: 0.1 to 20 mg/L.
Accuracy: +/- 1 percent of maxiumum range.
Operating temperature range: -5 deg. to +45 deg.C.
Minimum scale readability: 0.05 mg/L 02
Temperature-compensated, portable, and rugged for field and
laboratory use.
4.3 Calibration chamber, Yellow Springs Instrument Co., Model no
5075, or equivalent.
4.4 Pocket altimeter-barometer, capable of being read to at least
the nearest 5 millimeters of mercury. USGS warehouse supply stock
number 01321, or equivalent.
5. Reagents
5 1 Sodium sulfite solution: Dissolve 1 gram of sodium sulfite
(Na2S03) and a few crystals (about 1 mg) of cobalt chloride
(CoC12) in 1 liter of distilled water.
5.2 Reagents for Alsterberg-Azide method: See method I-1575. These
reagents are required when using calibration procedure 6.3.1B.
6. Procedure
6.1 Prepare oxygen meter for calibration in accordance with the
manufacturers instructions (Notes 1. 2. 3. 4. and 5).
Note 1. A number of precautions should be observed when using the
oxygen meter, particularly when the instrument is used for field
measurements. (l) Check mechanical zero before turning meter on
and adjust if necessary, (2) turn instrument on to any operational
position 1 hour before use to insure stability of operation; meter
should be left on until final measurement for day is completed.
(3) Check REDLINE and ZERO and adjust as needed; If meter will not
adjust to REDLINE recharge or replace batteries, (4) Protect
instrument from temperature changes that may result from exposure
to direct sunlight or contact with ice or cold wind.
Note 2. The DO probe requires periodic replacement of electrolyte
and membrane. Follow manufacturers directions but observe these
points: (l) There should be no bubbles of air behind membrane.
Electrodes with pressure compensators should have the pressure
compensation membrane moved in and out with the eraser end of a
pencil to insure that all bubbles are removed from pressure
compensation chamber. (2) the membrane should be smooth with no
wrinkles (3) excess membrane material should be trimmed to within
2 to 3 mm of the o-ring.
Note 3. O-rings stretch with use and must be replaced before they
become loose. A common cause of electrode malfunction is leakage
of electrolyte around the O-ring seal. To check for this problem,
rinse the probe unit to remove traces of electrolyte remaining
from membrane replacement. Place the probe in a 250-mL beaker of
distilled water; Measure the specific conductance with an
appropriate instrument. An increase in specific conductance
indicates leakage of the electrolyte around the o-ring. Check the
probe for physical damage. Replace the membrane and repeat the
test. It may be necessary to replace the o-ring also.
Note 4. Keep DO probe in water when not in use to prevent drying
out of the membrane.
Note 5. Periodically check the thermistor in the DO probe against
a certified mercury thermometer over the normal operating
temperature range of the instrument. If the two readings are not
in agreement a correction curve should be constructed or an
electronic adjustment should be made to the instrument by the
manufacturer or qualified service center.
6.2 Once daily, check that the instrument will read zero in a
dissolved-oxygen free solution. This may be accomplished by
inserting the probe into the sodium sulfite solution and measuring
the dissolved oxygen. If instrument reading exceeds 0.2 mg/L the
probe needs repair or the membrane and electrolyte need
replacement.
6.3 Calibrate the DO meter in accordance with one of the following
four procedures. Procedure A is the preferred procedure.
Procedures B, C and D may be used in situations where procedure A
cannot be used. These four procedures are given in order of
preference.
6.3.1A Air Calibration Chamber in water : The air calibration
chamber (Yellow Springs Instrument number 5075, or equivalent)
permits calibration of the oxygen meter at the temperature of the
water in which the DO content is to be measured, thereby
minimizing errors due to temperature differences. Dip the
calibration chamber into the water; pour out excess water, and
then insert DO probe into the wet chamber. This insures that the
air inside the chamber is saturated with water vapor. Caution: Be
sure that no water can leak into the calibration chamber and that
the membrane has no droplets of water adhering to it since this
would reduce the rate of oxygen diffusion through the membrane and
would produce erroneous results.
6.3.2A Place calibration chamber in the water in which the DO
content is to be measured. Allow 10-15 minutes for the temperature
of the air inside the chamber to equilibrate with the water.
Note 6. The calibration chamber (model 5075) is designed to allow
the membrane surface of the DO probe (Model 5739) to be at ambient
atmospheric pressure while in the chamber, but the chamber is not
large enough for the entire DO probe and pressure compensator to
fit inside. Since the pressure compensator must remain at
atmospheric pressure care must be taken to insure that the
pressure compensator is not submerged below the water surface if
method 6.3.1A is used.
6.3.3A Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.
6.3.4A Recheck the REDLINE and ZERO reading on the oxygen meter
and adjust if necessary.
6.3.5A Measure temperature in the calibration chamber to the
nearest 0.5 deg. C, using the thermistor in the DO probe.
6.3.6A Using the oxygen solubility table from section 7.2
determine the DO saturation value at the measured water
temperature and true atmospheric pressure. Apply a salinity
correction if appropriate.
Note 7. The solubility of oxygen in water decreases as salinity
increases. Oxygen saturation values may be corrected for salinity
using the correction factors based on either chloride
concentration or specific conductance. Correction factors based on
chloride concentration are given in the oxygen solubility tab1e in
section 7.2. Correction factors based on specific conductance are
also given in a table in section 7.2. The correction based on
specific conductance may be more useful since specific conductance
can be easily determined from a field measurement. The solubility
of oxygen at 25!C and 760 mm of mercury in water containing 5,000
mg/L of chloride is calculated as follows using the chloride
correction factors:
8.3 mg/L 5,000 mg/L Cl x 0.008 = 7.9 mg/L
(Saturation DO at 100 (Difference per
O mg/L chloride 100 mg/L chloride
and 760 mm Hg from from section 7.2)
oxygen solubility
table)
If, in the above example, specific conductance of the water was
15,000 micromhos per cm at 25 deg. C the calculation could be made
as follows using the specific conductance correction factors:
8.3 mg/L x 0.951 = 7.9 mg/L
(Saturation DO at (Correction factor from
O mg/L chloride and table in section 7.2 for
760 mm Hg from oxygen specific conductance of
solubility table) 15,000 micromhos and at
a temperature of 25 deg. C)
In both of the above examples the DO meter would be adjusted to
7.9 mg/L rather than 8.3 mg/L.
The YSI model 57 DO meter has a manual salinity correction built
into the instrument. Follow the manufacturer's instructions in its
use.
The salinity value required for use with the YSI model 57 meter
may be estimated from a field measurement of specific conductance
by the following equation:
Salinity, in parts per [Specific Conductance] - 1.025
thousand = 0.677 l000
6.3.7A Select the scale (0-10 mg/L or 0-20 mg/L) to be used for
the DO measurement and adjust CALIBRATION control until meter
reads the DO saturation value determined in 6.3.6A. Do not change
scales without recalibrating meter or verifying that identical
readings are obtained on both scales.
6.3.8A Remove probe from calibration chamber and place in water to
be measured. Move probe through water to insure a velocity of at
least 1 foot per second past the probe (alternatively, a
submersible stirrer, YSI Model 5491, or equivalent may be used) to
prevent localized depletion of DO at the membrane-water interface.
6.3.9A Switch meter to TEMPERATURE; after meter reading has
stabilized record temperature to the nearest 0.5 deg. C.
6.3.10A Switch meter to the proper DO scale; after meter reading
has stabilized record DO concentration to the nearest 0.1 mg/L.
6.3.1B Calibration by Winkler titration : The DO meter is
calibrated with distilled or deionized water in which the DO
concentration has been determined by the Winkler method.
6.3.2B Place 1 to 2 liters of distilled or deionized water in a
large container. The water should preferrably be near saturation
with respect to DO and the water temperature should be reasonably
close to the ambient temperature. These conditions should insure
that the DO concentration in the distilled water remains constant.
6.3.3B Place DO probe in the distilled water. Maintain a velocity
of at least 1 foot per second past the DO probe with a magnetic
stirrer or other means.
6.3.4B Check ZERO and REDLINE, and adjust if necessary.
6.3.5B Switch meter to the appropriate scale and adjust reading to
the approximate DO concentration with the CALIBRATION control.
6.3.6B After DO meter has stabilized determine the DO
concentration of two aliquots of the distilled water by the
Winkler titration procedure (Method I-1575). If results of the two
Winkler titrations do not agree within 0.1 mg/L, the titration
should be repeated.
6.3.7B Without delay recheck ZERO and REDLINE then adjust
CALIBRATION control until DO meter reads DO concentration
determined in step 6.3.6B. Meter is now calibrated and ready for
use.
6.3.8B Place DO probe in water to be measured. Allow sufficient
time for the probe to equilibrate with the water temperature. Move
probe through water at a velocity of at least 1 foot per second
(alternatively, a submersible stirrer, YSI Model 5491, or
equivalent may be used) to prevent localized depletion of DO at
the membrane-water interface.
6.3.9B Switch meter to TEMPERATURE; After meter has stabilized
record temperature to nearest 0.5 deg. C.
6.3.10B Switch meter to the appropriate DO scale. After meter has
stabilized record DO concentration to nearest 0.1 mg/L. The
alternate salinity correction procedure (See note 7) should be
applied if appropriate.
6.3.1C Calibration with Air Saturated Water: The DO meter is
calibrated against water which is saturated with oxygen at a known
temperature and true atmospheric pressure. Obtain approximately
1 liter of distilled water or water from the water body to be
measured. The temperature of the water used for calibration should
be close to ambient temperature so there is no heating or cooling.
6.3.2C Place DO probe and the calibration water in a large beaker
or open-mouth container and allow it to come to thermal
equilibrium. Shield beaker from direct sunlight and wind to
minimize temperature variations.
6.3.3C Aerate the water for 5-10 minutes. This is most readily
accomplished with a small battery-operated aquarium pump or minnow
bucket aerator and a short section of tubing. A gas diffusion
stone should be attached to the end of the tubing and placed at
the bottom of the beaker of calibration water.
Note 8. It is extremely important to insure that the water is
exactly 100 percent saturated with oxygen. An indication of 100
percent saturation can be determined as follows. Switch DO meter
to the 0-10 mg/L scale and adjust meter reading to approximately 8
mg/L with the CALIBRATION control. Observe meter while aerating
calibration water. When no change in DO reading on meter is
observed for a period of 4-5 minutes the water can be assumed to
be saturated.
6.3.4C Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.
6.3.5C Recheck the REDLINE and ZERO reading on the oxygen meter
and adjust if necessary.
6.3.6C Measure temperature of the calibration water to the nearest
0.5 deg. C.
6.3.7C Using oxygen the oxygen solubility table from section 7.2
determine the DO saturation value at the measured calibration
water temperature and true atmospheric pressure. Apply a salinity
correction if appropriate (Note 7).
6.3.8C Select the scale (0-10 mg/L or 0-20 mg/L) to be used for
the DO measurement. Turn off aerator and adjust CALIBRATION
control until meter reads the DO saturation value determined in
6.3.7C. Do not change scales without recalibrating meter or
verifying that identical readings are obtained on both scales.
Note 9. It is essential that calibration be completed with the
temperature of the calibration water at the value measured in
6.3.6C, otherwise the calibration water may be undersaturated or
oversaturated with oxygen. If the temperature changes or later
recalibration of the meter is necessary the calibration procedure
must be repeated beginning with step 6.3.2 C.
6.3.9C Remove probe from calibration container and place in water
to be measured. Move probe through water at a velocity of at least
1 foot per second (alternatively, a submersible stirrer, YSI Model
5491, or equivalent may be used) to prevent localized depletion of
DO at the membrane-water interface.
6.3.10C Switch meter to TEMPERATURE; after meter reading has
stabilized record temperature to the nearest 0.5 deg. C.
6.3.11C Switch meter to the proper DO scale; after meter reading
has stabilized record DO concentration to the nearest 0.1 mg/L.
6.3.1D Air Calibration Chamber in Air: This procedure is similar
to the procedure described in 6.3.1A, except that calibration is
performed with the air calibration chamber in air rather than in
water. This procedure can only be used with probes that have the
temperature sensing thermistor located adjacent to the membrane.
The probe must be capable of automatically compensating for
temperature changes. However, it is recommnded that this procedure
not be used if the air temperature differs from the water
temperature by more than 10 degrees. Also, the calibration chamber
must be shielded from direct sunlight and wind to avoid large
temperature fluctuations.
6.3.2D Wet the inside of the calibration chamber with water; pour
out the excess water and insert the probe into the wet chamber.
This insures 100 percent humidity. Allow sufficient time (10-15
minutes) for the DO probe and air inside the calibration chamber
to equilibrate. Observe precautions stated in 6.3.lD.
6.3.3D Read true atmospheric pressure from pocket altimeter-
barometer to the nearest 5 mm of mercury.
6.3.4D Recheck the REDLINE and ZERO readings on the oxygen meter
and adjust if necessary.
6.3.5D Measure temperature in the calibration chamber to the
nearest 0.5 deg. C using the thermistor in the DO probe.
6.3.6D Using oxygen the oxygen solubility table from section 7.2
determine the DO saturation at the measured temperature and true
atmospheric pressure. Apply a salinity correction if necessary.
(Notes 7)
6.3.7D Select the scale (0-10 mg/L or 0-20 mg/L) to be used for
the DO measurement and adjust CALIBRATION control until the meter
reading is at the DO saturation value determined in 6.3.6D. Do not
change scales without recalibrating meter or verifying that
identical readings are obtained on both scales.
6.3.8D Remove probe from calibration chamber and place in water to
be measured. Move probe through water at a velocity of at least 1
foot per second (alternatively, a submersible stirrer, YSI Model
5491, or equivalent may be used) to prevent localized depletion of
DO at the membrane-water interface.
6.3.9D Switch meter to TEMPERATURE; after meter reading has
stabilized record temperature to the nearest 0.5!C.
6.3.10D Switch meter to the proper DO scale; after meter reading
has stabilized record DO concentration to the nearest 0.1 mg/L.
7. Calculations
7.1 Read the concentration of dissolved oxygen in milligrams per
liter directly from the meter of the instrument.
7.2 The following table of values for the solubility of oxygen in
water exposed to water-saturated air should be used to calibrate
the DO meter. The table may also be used to determine dissolved
oxygen as percent saturation at the measured temperature and true
atmospheric pressure as follows.
DO (percent saturation) = Measured DO (mg/L)/saturation DO (mg/L)
x 100
8. Report
8.1 Report dissolved oxygen concentrations to the nearest
0.1 mg/L.
8.2 Report dissolved oxygen as percent saturation to the nearest
percent.
9. Precision
9.1 Precision data are not available for this method.
References
American Public Health Association and others, 1976, Standard
methods for the examination of water and wastewater (14th ed.):
New York, Am. Public Health Assoc., Inc., p. 446
Brown, Eugene, Skougstad, M. W. and Fishman, M. J., 1970, Methods
for the Collection and Analysis of Water Samples for Dissolved
Minerals and Gases: Techniques of Water-Resources Inv. of the U.S.
Geological Survey, book 5, ch. Al, p. 129.
Green, E. J. and Carritt, D. E., 1967, New Tables for Oxygen
Saturation of Seawater: Journal of Marine Research, p 140.